If the flight was operated by a third party, BA is in no place at all to follow this up as it is a legal issue within that charter airline. You wouldn't go to ASDA to complain about Tescos, would you? It may be in their interests to know but they will be limited in what they can do. In fact, All BA really can do is offer an apology - and it will take around 6 weeks to get one. They may pass on your email to the charter airline but with data protection they would not be told the outcome of an internal disciplinary.
With this nature of complaint, you are claiming sexual harrassment - this is serious, you can't put this kind of allegation into the same complaint system for not getting your choice of meal. You can't just write in and say so-and-so did this and so-and-so said that. You will have to request an internal investigation with the charter company directly (Astraeus, Titan, Monarch or whoever covered your flight). In all probability their legal department will contact you so you can make a statement. They will then suspend the whole flight crew until statements are taken from them. They will also need a statement from your colleague. After all of this, they will then decide what action, if any, to take. It's your word against theirs.
If you feel it's worth persuing, then go for it but remember you may be dragging innocent crew into something that they had nothing to do with. You cannot just write into a company and expect them to take disciplinary action against an employee just on your say so - you have to be prepared to back up your allegations.